A spectrum analyzer is a scanning receiver that displays power and modulation characteristics of input signals over a specific frequency band. The spectrum analyzer may cover an extremely broad frequency range, for example, 0 to 27 GHz. In the high frequency portion of the range from 2-27 GHz, a superheterodyne receiver is commonly used with a tunable bandpass filter for rejecting images and multiple responses. The bandpass filter is typically a YIG-tuned resonator filter.
YIG-tuned resonator filters comprise a yttrium iron garnet (YIG) sphere suspended between two orthogonal half loop conductors. The YIG material exhibits ferrimagnetic resonance. In the presence of an external DC magnetic field, the dipoles in the YIG sphere align with the magnetic field, producing a strong magnetization.
An RF signal applied to the input half loop conductor produces an alternating magnetic field perpendicular to the DC magnetic field. In the absence of the YIG sphere, the magnetic field is not coupled to the orthogonal output half loop conductor. The dipoles in the YIG sphere precess around the applied DC magnetic field at the frequency of the RF signal when the RF frequency is close to the resonance frequency of the dipoles. The resonance frequency for a spherical YIG resonator is: EQU f.sub.p =.gamma.(H.sub.O .+-.H.sub.a)
where H.sub.O is the strength of the applied DC field in oersteds, H.sub.a is the internal anisotropy field within the YIG material and .gamma. is the gyromagnetic ratio (2.8 MHz/oersted).
When an RF signal at or near resonance frequency f.sub.p is applied to the input half loop, the RF signal causes the dipoles in the YIG resonator to precess at the frequency of the RF signal. The precessing dipoles create a circularly polarized magnetic field rotating at the RF frequency in a plane perpendicular to the externally applied DC magnetic field. This rotating field is coupled to the output half loop conductor, inducing an RF signal in the output loop that, at the resonance frequency, is phase shifted 90.degree. from the input RF signal. Because the resonance bandwidth can be made fairly narrow, the YIG resonator makes an excellent filter at RF frequencies. The filter is tunable by varying the strength of the applied DC magnetic field.
YIG-tuned resonator filters typically include three or more YIG-tuned resonators connected in series to obtain a highly selective filter response. Each resonator includes a YIG sphere with input and output half loops. Additional functions may be incorporated into the resonator circuit. For example, a switch associated with the input resonator may be used to switch a low frequency input signal to a low frequency signal processing section of the spectrum analyzer. A harmonic mixer may be used to downconvert the input RF signal to an IF frequency. A tracking YIG-tuned filter-mixer is disclosed in U.S. Pat. No. 4,817,200 issued Mar. 28, 1989 to Tanbakuchi.
In order to obtain optimum performance from the YIG-tuned resonator filter, each resonator should be tuned to the same or nearly the same frequency, and the resonance frequencies should track over the frequency range of interest. Any departure from this requirement produces ripple within the passband of the filter and a generally degraded frequency response. In practice, it has been found that the intermediate resonators of a YIG-tuned filter do not track the input and output resonators, when a uniform magnetic field is applied to all the resonators. Specifically, the intermediate resonators are pulled down in frequency relative to the input and output resonators as the filter is tuned from the lower end of its frequency range toward the upper end.
The pulling of the intermediate resonators relative to the input and output resonators is caused by the double coupling loops used in the intermediate stages. The input RF signal is coupled to the input resonator using a single half loop. Similarly, the output RF signal is coupled from the output resonator using a single half loop. However, the RF signal is coupled to the intermediate resonators using double half loops, which have higher inductance than the single half loops. The higher inductance of the double half loops produces the frequency pulling of the intermediate resonators described above.
In order to insure that the resonators of a YIG-tuned resonant circuit track as a function of frequency, the intermediate resonator or resonators are tuned up in frequency, or the input and output resonators are tuned down in frequency, as the operating frequency increases. One prior art approach is to arrange the resonators in a circle between two magnetic polepieces and to use magnetic polepieces each having a tapered face. Since the magnetic field within the gap varies inversely with the distance between the polepieces, the magnetic field in the gap varies across the faces of the tapered polepieces. By rotating the polepiece, the middle resonator can be tuned with respect to the input and output resonators. In this prior art technique, the entire face of the magnetic polepiece is uniformly tapered. While this approach provides satisfactory performance for a filter having three resonators, its effectiveness decreases for filters with more than three resonators.
A second prior art approach is to use screws embedded in the magnetic polepiece underlying or overlying the input and output resonators. The screws are made of the same magnetic material as the polepiece. By adjusting the screws, the magnetic field applied to the input and output resonators can be varied. The disadvantages of this approach are that the cost of custom magnetic screws is high, the screws usually freeze inside the magnetic polepiece, the screw adjustment, typically on the order of 0.0003 inch, is very hard to control, and the screw can potentially contact and damage the resonator.
It is a general object of the present invention to provide improved YIG-tuned resonator circuits.
It is another object of the present invention to provide YIG-tuned resonator circuits wherein the resonators track over a frequency range of interest.
It is a further object of the present invention to provide a technique for adjusting tracking of YIG-tuned resonator circuits having four or more resonators.
It is yet another object of the present invention to provide YIG-tuned resonator circuits which are low in cost and in which tracking is easily adjusted.